This invention relates to drop marking equipment and, in particular, to nozzles used in such drop marking equipment or ink jet devices. Such devices employ inks which are supplied from a reservoir to a nozzle. The nozzle directs ink at a substrate to be marked. By use of a transducer, electrical energy is converted into mechanical energy, which is coupled to the ink in the nozzle. In one example of ink jet operation, the stream of ink ejected from an orifice at one end of the nozzle is broken up into a series of regularly spaced, discrete droplets which may be selectively given an electrical charge. In that type of drop marking device, those drops which receive a charge are deflected onto a substrate while those which are not charged are recovered and returned to the ink supply. In another type of droplet marking device, the transducer applies an impulse of energy to the fluid in the nozzle each instance that a droplet is needed.
As is well known by those in the art, the complexity of such ink jet nozzles contribute to cost and speed limitations. For example, it is often desirable to group together several such nozzles to permit high speed printing on a substrate which may be, for example, magazines, envelopes, labels, beverage cans on other products moving on a conveyor. It is not uncommon for ink jet nozzles in some applications to be spaced as closely as six per inch and thus the need there for a low cost, high quality, minaturized device is apparent.
A significant contributing factor to the complexity and cost of producing ink jet nozzles is the presence of both fluid and mechanical resonances in such assemblies which interfere with the nozzle's usefulnesses over the range of frequencies usually employed to form the ink droplets. Such resonances vary with the type of ink employed, temperature, and the geometric dimensions of the nozzle assembly. They are also significantly affected by the type of material used to manufacture the nozzle. As a result ink jet printers have required a variety of different nozzles to permit operation at different frequencies and for different kinds of inks.
Typically, ink jet nozzle assemblies have been manufactured from metal or glass materials and are acoustically "hard" meaning that they support acoustic resonances at certain frequencies with very little attenuation. The nozzle may vibrate in flexure, torsion, compression or all three imparting added mechanical energy to the ink stream at specific frequencies. Also a consideration in nozzle design is the fluid resonance, i.e., resonance in the ink contained within the nozzle body. If a fluid is confined in a chamber having a rigid wall, a standing wave is formed, in this case inside the fluid containing chamber. One standard nozzle design technique calls for configuring the nozzle assembly to have a mechanical resonance that is outside the operating frequency range of the nozzle, while the fluid chamber and ink are matched to have a fluid resonance in the operating frequency range. In that type of nozzle assembly, operation is restricted to frequencies substantially coincidental with the fluid resonance region because only in that region can energy be transmitted to the fluid efficiently and the droplets be formed reliably. As is well known according to acoustic principles involved in vibrating bodies, these nozzles have specific resonance frequencies for the fluids selected. The disturbing energy applied to the nozzle cannot be efficiently transmitted to the fluid to form droplets if the frequency selected for operation is not substantially coincidental with the resonance frequency of the selected fluid.
The present invention contemplates, at least in one aspect, proceeding contrary to accepted wisdom by designing nozzles without resonance so as to eliminate the antiresonance regions in the operating frequency range and thereby extend the operating frequency range of the nozzle. To do that, acoustically soft materials were sought so that resonances would be substantially unsupported. This permits only the disturbing energy created by an electromechanical transducer, for example, a piezoelectric crystal, operating at a selected frequency to be transmitted to the fluid.
In the prior art efforts have been made to overcome the difficulties which arise from fluid and mechanical resonances. These are discussed in U.S. Pat. Nos. 4,379,303, 4,349,830, and 3,972,474, for example. Typically, reduction of fluid resonance has been attempted by using either a labyrinth of small passages or by making the nozzle body as short as possible. In general, these procedures move portions of the resonances to higher frequencies (usually outside the operating frequency range). However, harmonics of the undesirable resonances remain and show up in the operating frequency range of the nozzle.
According to the present invention, a nozzle assembly is disclosed which employs an acoustically soft material which can overcome most or all of the disadvantages of present assemblies and which is more versatile than the latter because it provides additional advantages not heretofore obtainable. Specifically, according to the present invention, (1) the ink is electrically isolated from the transducer permitting the reference potential of the ink to be independently adjusted relative to the driving signal to the transducer, if desired; (2) the nozzle assembly can be formed by molding techniques and mass produced at low cost; (3) the operating frequency range of the nozzle is broadened by eliminating antiresonance regions; (4) electrolytic action can be controlled by use of an electrode and filter arrangement in the ink system including the nozzle.